Signals sent over optical fibers consist of sequences of binary ones and zeroes. A common modulation used for transmission of the binary sequences is NRZ (Non-return Zero) line code. In a NRZ transmission binary ones are transmitted using one physical condition and binary zeroes are transmitted using another physical condition. In the case of data transmissions over optical fibers light pulses are used to transmit the data. In a typical NRZ line code a light pulse sent by a laser signals a binary one whereas a weaker or no light pulse signals a binary zero.
Further, it is typically desired to add a management channel to the data channel. The management channel can be used to transmit various commands and settings through the system or be used to upgrade software. The management channel can typically be implemented by inserting management channel frames in the data channel. Another way is to send the management data on a separate channel.
Further, the international patent application WO 84/01679 describes a transmission system where an information channel is added to the traffic signal by means of pulse width modulation. The information channel can for example be used to transmit a voice signal.
There is a constant desire to improve the performance in transmission systems. Hence, there is a need for a transmission system with improved performance for transmitting management data together with a data signal.